The present application claims priority to Japanese Application No. P10-220694 filed Aug. 4, 1998, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to an optical writing type liquid crystal light valve apparatus and a producing method thereof. The present invention improves sensitivity with respect to a writing light and a spatial resolving power in the optical writing liquid crystal light valve apparatus.
2. Description of the Related Art
A liquid crystal light valve apparatus (LCLV) is an optical-optical image converter.
The light valve is such an apparatus that receives a light with low light intensity and reads an optical image by means of a light from another light source in real time so as to be capable of outputting the optical image.
The LCLV has been utilized as an application to a large-sized screen for military use and commercial use. For example, an LCLV, which was announced by Rodney D. Sterling of Hughes et al. in xe2x80x9cVideo-Rate Liquid Crystal Light-Valve Using an Amorphous Silicon Photo Detectorxe2x80x9d, SID, ""90 Digest, Paper No. 17A 2, pp327-329 (1990), as shown in a schematic cross-section of FIG. 1, is constituted so that a transparent electrode 2 is formed on a first transparent glass substrate 1, and a photoconductive layer 3 which is made of an amorphous silicon (herein after referred to as a-Si) and whose thickness is large and continuously uniform is formed thereon, and further a light shielding layer 4 made of CdTe or the like and a dielectric mirror 5 as an optical reflective layer is laminated thereon, and an alignment layer 6 is formed on the dielectric mirror.
Meanwhile, a second transparent glass substrate 7 is prepared, and a transparent electrode 8 and an alignment layer 9 are formed thereon similarly.
The first and second glass substrates 1 and 7 are opposed to each other with a gap of about several xcexcm with the sides where the alignment layers 6 and 9 are formed being directed inside, and liquid crystal is filled between the substrates 1 and 7 so that a liquid crystal layer 10 is formed. The LCLV is constituted in such a manner.
In this LCLV, the side of the second glass substrate 7 is set as an observation side for an optical image, and a reading light LR is irradiated on the glass substrate 7 vertically from the glass substrate 7 side by a polarized light. At this time, the reading light passes through the transparent electrode 7 and the liquid crystal layer 10 and is reflected on the dielectric mirror 5, and passes through the liquid crystal layer 10 and the glass substrate 7 again so as to be emitted to the outside. In such a manner, the reflected light can be observed.
On the contrary, while an alternating voltage AC is being applied between the transparent electrodes 2 and 8 in the state that the reading light LR is irradiated, a writing light LW is irradiated from the first glass substrate 1 side. Then, the writing light LW passes through the first glass substrate 1 and the transparent electrode 2 and is irradiated onto the photoconductive layer 3, and the photoconductive layer 3 is activated on this irradiated portion so that electron-hole pairs are generated. As a result, electrostatic capacity of the photoconductive layer 3 is increased, a resistance value thereof is decreased and voltages sandwiching the liquid crystal layer are increased according to a pattern corresponding to an irradiating pattern and the intensity of the writing light LW. This spatial change in the voltage becomes a change in a direction of liquid crystal molecules, and this change causes birefringence and rotation of the reading light LR which passes the liquid crystal layer, and the azimuth (polarization) of the reading light LR is modulated. Therefore, when the reading light emitted from the second glass substrate 7 is finally allowed to pass through a deflecting plate, the reading light can be observed as a change in light quantity. In other words, an optical image according to a pattern of the writing light LW, namely the optical image can be observed from the side of the second glass substrate 7.
Here, the light shielding layer 4 is arranged between the dielectric mirror 5, namely, the light reflective layer and the photoconductive layer 3, so that even a slight quantity of the reading light, which has passed through the light reflective layer, is absorbed by the light shielding layer 4. Namely, the light shielding layer 4 is disposed so as to avoid that the reading light LR reaches the photoconductive layer 3 to activate the photoconductive layer 3 and to generate an image other than a writing light, in other words, noises are produced.
Incidentally, in the above LCLV, in order to obtain high sensitivity, it is desired to make a voltage, which is applied to the liquid crystal layer according to a change in the resistance value of the photoconductive layer 3, to be maximum. The maximum voltage is achieved when the impedance of the photoconductive layer 3 on which the light is not irradiated and the impedance of the liquid crystal layer 10 satisfy the following condition.
That is, impedance, which is generated by a parallel circuit of equivalent capacity and bulk resistance of the photoconductive layer 3, is set so as to be substantially equal with or exceed impedance, which is generated by a parallel circuit of capacity and resistance of the liquid crystal layer 10 (this condition is referred to as a balancing relationship).
Then, the balancing relationship can be realized by, concretely, setting a film thickness of the photoconductive layer made of a-Si to become about 30 xcexcm.
The film thickness of 30 xcexcm is required because when the balancing relationship between the impedances of the photoconductive layer 3 and the liquid crystal layer 10 is tried to be set, a dielectric constant of the photoconductive layer 3 due to the a-Si film is higher than a dielectric constant of the liquid crystal layer 10.
However, when the photoconductive layer made of the a-Si layer has a thickness up to 30 xcexcm in such a manner, electric charges generated on the photoconductive layer are easily diffused in an adjacent area because of the incidence of the writing light.
Namely, an ideal LCLV is constituted so that its photoconductive layer has high resistance such that electric charges generated due to light irradiation can be prevented from diffusing in a lateral direction. However, if the thickness of an a-Si photoconductive layer becomes up to about 30 xcexcm, sufficiently high resistance cannot be obtained, and thus the electric charges in the lateral direction (surface direction) easily diffuse. As a result, spatial contrast is lowered and resolution is lowered.
In order to avoid such inconvenience, such a trial was carried out that dopant was added to an a-Si layer composing the photoconductive layer so that resistivity was improved. In this method, when the a-Si film is deposited, since this film has a property such that n-type dopant is originally generated, the n-type dopant is canceled by doping p-type dopant such as boron.
However, according to this method, since the effect of addition of the dopant is extremely great and the occurrence of the n-type dopant varies every time the a-Si film is deposited, it is actually very difficult to accurately dope the p-type dopant for setting desired resistivity. As a result, the production cost is increased, and yield is lowered.
Accordingly, a split structure, such that a photoconductive layer is separated completely at every pixel, is suggested (U.S. Pat. No. 5,076,670). This separation is executed by pattern etching using photo-lithography, for example, but as mentioned above, when the photoconductive layer having a large thickness up to 30 xcexcm is pattern-etched, time required for the work becomes longer, and further it is difficult to clearly pattern the photoconductive layer. Moreover, an insulating material is embedded into grooves between the separated photoconductive layers, but the work for embedding the insulating material into the grooves, whose aspect ratio (a ratio of a groove depth d to a width w: d/w) is high, is complicated. Thus causes problems such that the cost becomes high and satisfactory reliability is not obtained.
In addition, it is suggested that a photoconductive layer made of a-Si of high dielectric constant is formed selectively into holes which are formed on pixel formed areas of an insulating layer with low dielectric constant (U.S. Pat. No. 5,612,800). However, also in this case, since the dielectric constant of the insulating layer is about ⅓ of the dielectric ratio of a-Si, it is considered that the photoconductive layer requires a thickness of 10 xcexcm which is about ⅓ of the conventional thickness. For this reason, the film thickness cannot be made to be sufficiently small. Moreover, in this case, since the photoconductive layer is formed into a thin pillar shape, an effective cross section of a light receiving section of the photoconductive layer is decreased to about 4%, and thus the sensitivity for a writing light is lowered greatly. Therefore, in order to supplement this situation, a microlens array, where microlenses for converging a writing light onto the pillars of the photoconductive layer are arranged at respective pillars, is arranged. However, in this case, since the step of producing the microlens array and the step of locating the microlens array are complicated, and high accuracy is required, mass production is hindered and the cost becomes higher.
In addition, as shown in a schematic sectional view in FIG. 2, there suggests a structure such that a conductive layer 11 with high reflectance and low resistivity is formed selectively so as to oppose a transparent electrode 2 across a photoconductive layer 3, and an insulating layer 12 is formed between the adjacent conductive layers 11 (in FIG. 2, the same reference numerals are given to portions corresponding to those in FIG. 1 and the description thereof is omitted).
In this case, the collection of electric charges generated in the photoconductive layer 3 due to the conductive layer 11 are promoted, and a reading light which passed through a dielectric mirror 5 is reflected so as not to enter the photoconductive layer 3 without providing the light shielding layer 4 in FIG. 1.
However, also in the case of this structure, since an opposing area between the conductive layer 11 and the transparent electrode 2 opposing each other across the photoconductive layer 3 becomes larger, it is necessary for reducing capacity of the photoconductive layer to increase a thickness of the photoconductive layer 3 as usual.
In addition, from the viewpoint of preventing the electric charges between the pixels from diffusing, there suggests a structure such that only a part of the photoconductive layer is separated by an element separation insulating layer area between the elements, and thus the elements are not separated completely (U.S. Pat. No. 4,913,531). However, according to this method only, it is expected that the diffusion of electric charges generated in the photoconductive layer by a writing light to an adjacent element area can be reduced slightly, but a problem of planar lowering of resolution cannot be solved completely. Moreover, there still exists a problem that a photoconductive layer with large thickness is required.
In addition, Japanese Patent Application Laid-Open No. 9-197432 discloses a structure such that an intermediate electrode is arranged so as to face a transparent electrode across a photoconductive layer, and the intermediate electrode is composed by split electrodes, and an area of the transparent electrode which faces the split electrodes is set to be small so that capacity concerning the photoconductive layer is reduced. However, in this case, since an opposing area between the split electrodes and the transparent electrode is determined by an area of the transparent electrode, for example, a width, when this area is set to be sufficiently small, distributed resistance of the transparent electrode is increased, and thus a problem of responsibility arises occasionally.
In addition, in order to prevent a reading light from entering an a-Si photoconductive layer on the writing side through a dielectric mirror and from influencing an input image by the incident light, as mentioned above with reference to FIGS. 1 and 2, a light shielding layer 4 made of CdTe is provided.
However, since this CdTe layer has a photoconductive characteristic originally, when a leaked reading light is made incident on the CdTe light shielding layer, electron-hole pairs are generated in the CdTe light shielding layer. As a result, since the light shielding layer shields the electric charges which are generated in the photoconductive layer by a writing light, spatial resolution of an image on the reading side is lowered.
In addition, the CdTe is a material having toxicity, and thus its handling is complicated and disposal of it causes a problem. Accordingly, from the viewpoints of these problems and of a rise in the production cost and earth environmental protection, the use of CdTe is desired to be avoided.
The present invention has been achieved in order to solve the above-mentioned problems. In an optical writing type liquid crystal light valve apparatus, a voltage applied to a liquid crystal layer is made to be as maximum as possible, and a photoconductive layer is thinned so as to have a thickness of about 1 to 2 xcexcm which is required for absorption of a writing light into the photoconductive layer, and a light shielding layer composed of a CdTe film can be omitted.
Namely, it is an object of the present invention to provide an optical writing type liquid crystal light valve apparatus which is capable of realizing high sensitivity and high resolution, and improving productivity such as reduction in difficulty of the producing process and the production cost, and being suitable for earth environmental protection, and to provide a producing method thereof.
An optical writing type liquid crystal light valve apparatus of the present invention is constituted so as to have at least first and second transparent substrates, a photoconductive layer, first and second electrodes arranged so as to sandwich the photoconductive layer, an optical reflective layer, a liquid crystal layer, and a third electrode.
The second electrode is composed of split electrode sections which are obtained by dividing the second electrode into plural electrode sections, and at least one portion of the first electrode is provided at a position which faces gaps between the split electrode sections. An opposing area between the first and second electrodes is set so as to be smaller than an area of the second electrodes.
In addition, similarly to the above apparatus, an optical writing type liquid crystal light valve apparatus of the present invention is constituted so as to have at least first and second transparent substrates, a photoconductive layer, first and second electrodes, an optical reflective layer, a liquid crystal layer and a third electrode, in which the first and second electrodes are arranged on a same surface side of the photoconductive layer.
In addition, the second electrode is composed of split electrode sections which are obtained by dividing the second electrode into plural electrode sections, and the first electrode is arranged between the split electrode sections.
In addition, similarly to the above apparatuses, an optical writing type liquid crystal light valve apparatus of the present invention is constituted so as to have at least first and second transparent substrates, a photoconductive layer, first and second electrodes, an optical reflective layer, a liquid crystal layer and a third electrode, in which the first and second electrodes are arranged so as to sandwich the photoconductive layer or arranged on a same surface side of the photoconductive layer.
The second electrode is composed of split electrode sections which are obtained by dividing the second electrode into plural electrode sections, and the split electrode sections are composed so as to have an opposing electrode section which faces the third electrode across the liquid crystal layer and a contact section which is electrically connected with the opposing electrode section and comes in contact with the photoconductive layer. The contact area of the contact section with the photoconductive layer is set so as to be smaller than an area of the opposing electrode section.
In addition, a method of producing an optical writing type liquid crystal light valve apparatus according to the present invention having at least first and second transparent substrates, a photoconductive layer, first and second electrodes which are arranged so as to contact with the photoconductive layer, an optical reflective layer, a liquid crystal layer and a third electrode, the second electrode being composed of plural split electrode sections, the method includes the step of forming an electric charge diffusion restricting area, for selectively restricting diffusion of electric charges, on the photoconductive layer.
In the above structures, an alternating voltage is applied between the first and third electrodes, and a writing light is allowed to enter from the side of the first transparent substrate, and a polarized reading light is allowed to enter from the side of the second transparent substrate. As a result, electron-hole pairs are generated in the portion of the photoconductive layer on which the writing light is irradiated, and the capacity of the photoconductive layer is increased according to the intensity of the irradiated light so that resistance is lowered. For this reason, impedance between the split electrode sections of the second electrode and the first electrode on the writing light irradiated portion under the irradiated portion of the writing light for the photoconductive layer is lowered, and the voltage between the split electrode sections and the third electrode which sandwich the liquid crystal layer is increased. Namely, the voltage is applied to the liquid crystal layer according to the intensity on the writing light irradiated portion, and birefringence and rotation of the reading light are executed. When the reading light emitted from the second transparent substrate is detected through a deflecting plate, an optical image can be obtained by the reading light of a pattern according to a pattern of the writing light.
In the optical writing type liquid crystal light valve apparatus of the present invention, when the substantial opposing area between first and second electrodes arranged via the photoconductive layer is made small, the opposing area is reduced or when the substantial distance between the first and second electrodes is increased, the capacity of the photoconductive layer is reduced. As a result, the thickness of the photoconductive layer can be small in order to set a relationship where the impedance of the photoconductive layer is balanced with the impedance of the liquid crystal layer.
At least one portion of the first electrode is arranged at the position which faces the gaps between the split electrode sections of the second electrode, or arranged between the split electrode sections on so as to form a plane. The contact sections which contact with the photoconductive layer with a small area are formed on the split electrode sections. As a result, the opposing area between the first and second electrodes can be substantially reduced without reducing the area, i.e., width of the first electrode.
In addition, the method of producing the optical writing type liquid crystal light valve apparatus according to the present invention has the step of forming an electric charge diffusion restricting area, for selectively restricting diffusion of electric charges, on the photoconductive layer. When the electric charge restricting area is formed on the photoconductive layer in such a manner, the diffusion of the electric charges in the surface direction can be avoided efficiently. The lowering of the contrast between the pixels due to giving/receiving of the electric charges between the adjacent pixels can be avoided efficiently, and thus image quality can be improved.